The filter API is intended for cross cutting concerns that are applied indiscriminately to all routes. For example, here are some common use cases for filters:

Logging/metrics collection

GZIP encoding

Blanket security filters

In contrast, action composition is intended for route specific concerns, such as authentication and authorisation, caching and so on. If your filter is not one that you want applied to every route, consider using action composition instead, it is far more powerful. And don’t forget that you can create your own action builders that compose your own custom defined sets of actions to each route, to minimise boilerplate.

Let’s understand what’s happening here. The first thing to notice is the signature of the apply method. It’s a curried function, with the first parameter, next, being a function that takes a request header and produces a result, and the second parameter, rh, being a request header.

The next parameter represents the next action in the filter chain. Invoking it will cause the action to be invoked. In most cases you will probably want to invoke this at some point in your future. You may decide to not invoke it if for some reason you want to block the request.

The rh parameter is the actual request header for the request.

The next thing in the code is a function that logs the request. This function takes a PlainResult, and after logging the request time, adds a header to the response that records the Request-Time, and returns that result.

Finally the next action is invoked, and pattern matched on the result it returns. A result can either be a PlainResult or a AsyncResult, an AsyncResult is a result that will eventually be redeemed as a PlainResult. In both cases, the logTime function needs to be invoked, but is invoked in a slightly different way for each. Since if it’s a PlainResult the result is available now, it just invokes logTime directly. However, if it’s AsyncResult the result is not yet available. So, the logTime function is passed to the transform method to be invoked later, when the PlainResult is available.

Filters wrap the action after the action has been looked up by the router. This means you cannot use a filter to transform a path, method or query parameter to impact the router. However you can direct the request to a different action by invoking that action directly from the filter, though be aware that this will bypass the rest of the filter chain. If you do need to modify the request before the router is invoked, a better way to do this would be to place your logic in Global.onRouteRequest instead.

Since filters are applied after routing is done, it is possible to access routing information from the request, via the tags map on the RequestHeader. For example, you might want to log the time against the action method. In that case, you might update the logTime method to look like this:

The key difference here, apart from creating a new EssentialAction to wrap the passed in next action, is when we invoke next, we get back an Iteratee. You could wrap this in an Enumeratee to do some transformations if you wished. We then map the result of the iteratee, and handle it with a partial function, in the same way as in the simple form.

Although it may seem that there are two different filter APIs, there is only one, EssentialFilter. The simpler Filter API in the earlier examples extends EssentialFilter, and implements it by creating a new EssentialAction. The passed in callback makes it appear to skip the body parsing by creating a promise for the Result, and returning that in an AsyncResult, while the body parsing and the rest of the action are executed asynchronously.